A common failure mode of downhole hammers, percussion tools, and other multi-component downhole tools involves fatigue breakage of the lower housing, or in the case of a tool connected to a drill bit, breakage of the bit, or the bit connection shank. This failure mode can result in loss of some or all of the lower tool components or in the case of a tool connected to a drill bit, the loss of the bit, the bit and shank, or alternatively, the bit, the shank, and the lower housing of the tool being left in the hole after the failure, thereby creating a need to fish these parts out of the hole. Various methods, including various configurations of split retainer rings, have been used in the past to prevent the complete loss of the components below the failure point so that the complete assembly, though broken, can be pulled from the hole. An example of a downhole percussion tool that is the subject of at least one exemplary embodiment of this invention is described and illustrated in U.S. Pat. No. 7,377,338, entitled “Downhole Percussion Tool,” issued to Bassinger on May 27, 2008, which is incorporated by reference in its entirety herein.
An example of a prior art retaining ring is described and illustrated in U.S. Pat. No. 5,390,749, entitled “Apparatus for Positioning A Split Retaining Ring In A Down-Hole Percussive Drill,” issued to Lyon on Feb. 21, 1995 (“Lyon”), which is incorporated by reference in its entirety herein. An alternative example of a prior art split retainer ring is described and illustrated in U.S. Reissued Pat. No. RE 36,002, entitled “Transmission Sleeve For A Down Hole Hammer,” issued to Elsby et al. on Dec. 22, 1998 (“Elsby”), which also is incorporated by reference in its entirety herein. Another example of a prior art retaining ring is described and illustrated in U.S. Pat. No. 4,924,948, entitled “Shock Absorbing Bit Retaining Ring,” issued to Chuang et al. on May 15, 1990 (“Chuang”), which also is incorporated by reference in its entirety herein.
According to Lyon, the problems inherent in prior art split rings are explained. Lyon states, “Down-hole drills typically use a set of split retaining ring halves to retain the drilling bit within the casing of the drill. These rings are usually flat and split so they can be axially sandwiched and retained between internal drill parts, i.e. the chuck and the bit bearing. Due to high levels of vibration and shock in the bit end of the down-hole drill, conventional split retaining rings are prone to move radially back and forth within a space provided by the split between the halves. Such movement of rings can cause a number of problems, namely: 1. rubbing of the rings against the bit causing the bit to crack; 2. incomplete contact between the ring halves and the bit contact shoulder; 3. damage to the contact zone on the bit; and 4. high stresses in the ring halves.” See Lyon, col. 1, ll. 12-28 (emphasis added).
It is clear from Lyon's description that prior art rings are relatively loosely fitted into the space provided. Lyon describes how the rings of his invention are manufactured and states, “The separate ring halves are produced from one hollow, cylindrical body which is first tapered at the top and bottom, and thereafter diametrically cut into two pieces. The kerf of the cut represents the amount of arc distance that will be present between the ring halves, as assembled in a drill.” See Lyon, col. 4, ll. 11-16 (emphasis added). Lyon's solution to the above problems is to taper the top and bottom of the split ring to match tapers in the top of the lower housing and in the bottom of the bit bearing to drive the split rings outward to the inner diameter of the tool outer case. Thus, although a kerf exists between the two ring halves, Lyon is attempting to reduce the radial back and forth movement of the rings by using tapers on the top and bottom surface of the rings and matching tapers on adjacent components. However, Lyons does not address the movement allowed by the “kerf of the cut.” Additionally, the presence of the kerf reduces or eliminates the ability of the split ring to contribute stabilizing guidance to the traveling bit shank.
Uniformly within the prior art the expedient of saw cutting the split ring from a single turned ring (even with a highly tensioned, sharp and slowly applied mechanical saw blade) has resulted in a typical minimum “kerf′ width of 0.032” creating the problems of point loading, wear, play, stress, cocking and damage as described or alluded to by Lyon. A further result of the existence of the prior art kerf width was a desire in the prior art to loosely tolerance the fit of the retainer ring set to the inner case diameter and the outer shank diameter since the “play” allowed by the kerf width could cause binding and galling of these surfaces. Additionally, the significant kerf width and loose tolerances caused the prior art retainer rings to offer less resistance to lateral displacement of the sliding shank resulting in non-productive and eventually destructive vibration.
What is needed or desired is a retaining ring for downhole tools which, though split for the purposes of tool assembly, acts when assembled as a solid (or near solid, monolithic, or unitized) ring, tightly yet slidably engaged with the inner case diameter and the outer shank diameter.
The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments.